The present invention relates to a recording disk data playback apparatus having a capability for playback of both analog and digital recorded data from recording disks, and in particular to a recording disk data playback apparatus equipped with improved switchings circuits for enabling connection of various combinations of audio signals, produced by demodulation of playback analog and digital signals, to two or more sets of output terminals.
Various types of recording disk data playback apparatus, sometimes referred to as disk players, have been developed. Until recently, such players were divided into two basic types. One type is utilized only for playback of video disks on which video and audio data are recorded in the form of analog signals. Such disks are sometimes referred to as LDs, and this abbreviation will be used for these in the following. With such disks, the video and analog signals are utilized to frequency-modulate a high-frequency carrier signal, and the resultant modulated signal is recorded on the disk. The other type of disk player is utilized for playback of digital audio disks, sometimes referred to as compact disks or CDs, and the latter abbreviation will be used hereinafter for such disks. CDs have audio data recorded thereon in the form of a digital signal of PCM (pulse-code modulation) type, i.e. a carrier signal is modulated by an encoded digital signal representing the audio data, and the modulated signal is recorded on the disk. However in recent years, a new type of recording disk (designated in the following as LDD) has been developed, as described in Japanese Pat. No. 58-45780) whereby an audio signal which has been digitized, e.g. by a method such as PCM, is converted into a pulse train of form suitable for disk recording, e.g. by applying the EFM (eight-to-fourteen) technique, and this pulse train signal is then superimposed upon a signal which has been produced by FM modulation of a high-frequency carrier by a video signal and an audio signal. The signal which results from this superimposition of the pulse train signal upon the modulated carrier is recorded on the disk. With the latter method, the audio signal is generally separated into two channels, e.g. corresponding to the stereophonic right and left channels, with 2.3 MHz and 2.8 MHz audio carriers being respectively frequency modulated by the two audio channel signals. The frequency spectrum of the recorded signal is such that the the sync tip portions of the video signal correspond to a frequency of 7.6 MHz, the pedestal level to 8.1 MHz, and the white peak level to 9.3 MHz. If the EFM technique is used to record the audio digital signal, then the frequency spectrum of the pulse train will extend from 3T to 11T, where T is the bit period of the PCM signal, 3T corresponds to a pulse frequency of approximately 720 KHz, and 11T is the maximum pulse width and corresponds to a frequency of approximately 200 KHz. This pulse train signal is superimposed on the main video carrier at a level which is approximately 1/10 of the carrier level, or less. Amplification and slicing close to the zero-crossing points are then performed to produce a pulse-width modulated signal, which is used as the recording signal.
With video and audio signals recorded on a disk by the method described above, the frequency spectrum of the RF signal which is produced from the disk will be as shown in FIG. 1. Here, A denotes the digitized audio signal component, B denotes the audio FM signal component, C denotes the color information component of the video FM signal component, and D denotes the brightness component of the video FM signal component.
A very wide dynamic range, e.g. 90 dB or higher is provided by a digitized audio signal with such a system. Thus, a substantial improvement in acoustic fidelity can be attained, by comparison with recording and playback of audio signals using frequency modulation.
A digital demodulator system for demodulation of a digital signal containing audio data, such as that produced by playback of a digital audio disk, generally incorporates a memory, into which digital data is temporarily stored and then read out, to eliminate the effects of time-axis deviations, e.g. jitter. Prior to being subjected to digital/analog conversion, the digital signal containing the audio data is temporarily written into this memory, with the write-in operation being performed in synchronism with a write-in clock signal of specific frequency. Shortly thereafter, the store digital data is read out of the memory, in synchronism with a read-out clock signal. With a prior art type of recording disk data playback apparatus capable of playback of disks having digitally recorded audio data, the frequency of the read-out clock signal is held fixed, while the frequency of the write-in clock signal is synchronized with that of a playback clock signal which is derived from the digital signals output from the disk. The phase of the playback clock signal is determined by the rate of rotation of the recording disk.
During playback of an LDD, when a digital signal derived from the playback signal (produced by the video playback apparatus) is demodulated, the playback digital audio signal will already be synchronized in phase with a reference frequency signal which is utilized for video synchronization. In the case of a demodulator utilized in a prior art type of CD player apparatus, a separate reference frequency signal is used to produce the read-out clock signal, used to read out temporarily stored digital audio data (which was previously output from the PCM demodulator) from the memory as described above. data. If the above techniques were to be employed in a recording disk data playback apparatus capable of playback of both LDDs and CDs, then it will be apparent that two reference frequency signals would be necessary in order to generate the memory write-in and read-out clock signals, and if a slight phase deviation should arise between these two reference frequency signals, a time axis deviation will occur between the playback video signal and the demodulated playback audio signal.
In addition, the speeds of rotation of video disks and digital audio disks during playback are different, so that it is necessary to use two different spindle motors to drive the two types of disk. In order to ensure precise control of the speed of rotation of the disk, a spindle servo system is employed in a recording disk data playback apparatus. In the case of a CD player, the spindle servo system is controlled by a fixed-frequency clock signal which is produced from the read-out clock signal described above. On the other hand, in the case of a video disk playback apparatus, the spindle servo system is controlled by a horizontal sync signal which is contained in the playback FM video signal, so that the configurations of the spindle servo systems are different for the two prior art types of disk playback apparatus.
Thus, in order to develop a playback apparatus which will be capable of playback of both CDs and LDDs and which can utilize a single demodulator system in common for both CD and LDD playback, with low manufacturing costs, it is necessary to overcome the various problems described above.